Devices for introducing a freezable liquid into the exhaust gas system of a motor vehicle are known. For example, a known device meters an aqueous urea solution for NOx reduction into the exhaust gas. The urea solutions used for this purpose are composed of distilled water and 32.5% urea. This solution freezes at −11° C. If the motor vehicle is switched off and the temperature of the liquid drops below −11° C., the solution in the tank freezes and the liquid may expand by up to approximately 11% in the frozen state.
It is also known in this context to monitor the content of the freezable liquid in the device, in particular in the tank, by means of a sensor. This sensor serves to determine the concentration and/or the filling level of the freezable liquid in the tank and is arranged at least partially in the tank, e.g., on the floor thereof. In this context, the sensor can have a separate floor which is connected to the floor of the tank, for example welded thereto, or the parts of the sensor can be arranged directly on the tank floor.
In particular, in this context an ultrasonic sensor may be used, said sensor arranged to determine the concentration and/or the filling level of the aqueous urea solution mentioned above. In this context, the concentration can be determined from the characteristic propagation time of the liquid through which the radiation passes (speed of sound in the medium). Typically, the corresponding ultrasonic transducer emits an acoustic wave which is reflected at one or more reflection surfaces arranged in the tank and is converted again at the transducer, serving here as a transmitter and as a receiver, as an electrical signal and used further. The distance between the transducer and the reflector (or the reflectors from one another) determines the accuracy of the measuring system.
If the liquid in the tank is frozen, the mass of ice which is formed can press on the part of the sensor located in the tank and, in particular, damage the references (reflectors) or all of that part of the sensor which is arranged in the tank. These forces can increase further in the case of a tank which has semi-thawed (lumps of ice) during travel as a result of acceleration and sloshing. In order to prevent this, the part of the sensor which is located in the tank may be provided with a dome-shaped ice protector which is attached to the floor of the sensor or of the tank. This usually semi-spherical arrangement allows the ice to slide over the protector and therefore not damage the part of the sensor which is arranged in the tank. The forces that can occur as a result of the ice which is caught in the protector are negligible.
An ultrasonic sensor in which the transmitter/receiver is arranged on the underside of the tank floor is commercially available. On the floor of the actual tank there is a plate (flange) on which the other parts of the sensor are arranged. This involves a mirror for deflecting in the horizontal direction the sound waves which are emitted vertically from the transmitter, two reflectors and a further mirror with which sound waves are deflected vertically upward. The ice protector of this sensor is attached to the floor plate of the sensor and has, above the second mirror, an opening so that the sound waves can be emitted upward through the opening into the tank and can be reflected back by the surface of the urea solution. In this embodiment, the ice protector is therefore essentially closed with the exception of the opening mentioned above.
However, with such a sensor and with such an ice protector, bubbles can form under the ice protector, specifically when refueling or as a result of outgassing of the gases (atmospheric oxygen) which are dissolved in the liquid (urea solution), which bubbles can collect the air under the dome or cap similarly to a diving bell and prevent measurement of the propagation time or temporarily “blind” the sensor until the bubbles become detached or dissolved. The time until the renewed readiness for measurement can range here between seconds and several hours. Since the ice protector is also a separate part, additional costs arise due to material costs, manufacturing costs, fabrication costs, logistical costs and assembly costs.
DE 103 31 566 A1 discloses a flow medium level sensor which has a housing component which is of hollow-cylindrical design and has the purpose of covering components of the sensor. Further measuring arrangements for determining a filling level and/or a concentration of a liquid are known from DE 10 2011 013 687 A1, DE 10 2012 205 640 A1 and DE 10 2011 089 685 A1.